The invention belong to the branch of electronic devices and more particularly solves the problem of the recognition of a digital message coded in NRZ code ("no return to zero") and rhythmed by a given frequency.
The NRZ code which is often used in digital transmission is a binary code (hence a code having two levels) rhythmed by a clock, in which the levels, respectively 1 and 0, are maintained for successive bits having the same value (1 or 0), a change in level therefore taking place for two successive bits having different values. In the NRZ code, the duration of a bit is equal to the period of the clock defining the rhythm. It is necessary, for a message thus coded to be decoded, to know the frequency and the phase of the original rhythm. An article published in IEEE Transactions on Communications Vol. COM 20 Nr. Apr. 2, 1972, pages 94 to 104, describes a system for reconstituting the rhythm for an NRZ message, from the received message exclusively, and wherein the contingent errors due to the transmission of the message are taken into account by an appropriate statistical treatment.
In the known system, if the frequency of the message is known, an attempt is made to lock the phase of a local oscillator to the phase of the supposed NRZ message. The fact that a locking is obtained could indicate that there is a message. Practice has shown that such a principle gives a satisfactory result for recognition of the message only when the signal-to-noise ratio is high and when the frequency of the rhythm of the message is very stable. Now, these two conditions are rarely fulfilled in the case of long-distance transmission and more particularly in the case of a transmission by satellite.